The invention relates to a colour selection means for colour display tubes, comprising a mask and a frame, which frame includes a plurality of interconnected parts each forming a side of the frame, whereby at least two opposite frame parts are each formed by a pipe having a first pipe side which has an accurately defined edge, a second pipe side which is connected to the first pipe side along a path which does not project beyond the edge of the first pipe side, and at least one third pipe side which is located between the first and the second pipe side, the mask closely engaging the edge of the first pipe side, whereby the mask is stretched under tension and connected to at least two of the interconnected parts of the frame.
The invention also relates to a colour display tube provided with a colour selection means of the type referred to above.
A colour selection means as described above is disclosed in European Patent Application EP-A-0599400. The colour selection means described therein consists of a frame and a mask. The frame is built up from separate frame parts, namely the pipes, and the mask is stretched over the frame and secured to it. Such a construction can be used for the development of flat colour display tubes. In EP-A-0599400 only the frame construction is disclosed; the assembly of mask and frame is not described.
Colour display tubes are usually provided with a colour selection means to shadow the electron beams originating from the three electron guns mounted in the neck of the tube, so that each beam excites only electroluminescent material of one colour that is deposited on the inside of the panel. This colour selection is achieved by applying, for instance, a shadow mask in the tube. This mask comprises a large number of apertures which, in most cases, are arranged in a striped pattern or a dotted pattern. Conventional colour display tubes have a curved faceplate, in most cases it resembles either a spherical or a cylindrical surface. Recently, more and more colour display tubes tend to have a(n) (almost) flat faceplate. As a consequence, also the colour selection means will become flatter and flatter. The existing techniques to manufacture a colour selection means have shortcomings in the case of a substantially flat shadow mask, so that new ways of constructing suspensions for these shadow masks must be found.
One of the important performance-related issues in present-day colour display tubes is the doming behaviour. In fact, doming is the discolouration of the display due to local heating of the shadow mask. When the mask is heated, it will expand and, as a consequence, the electron beams will not impinge on the appropriate electroluminescent material on the panel. This misregistration causes a lack of the relevant colour, or even worse, electroluminescent material of the wrong colour is excited.
It is an object of this invention to provide a colour selection means construction in which the mask is connected to the frame in such a way that the requirements for obtaining a substantially flat colour display device are met.
According to the invention, a colour selection means of the type described in the opening paragraph is characterized in that the mask is connected to the frame by mechanical connection means that provide a flexible construction.
The invention is based on the recognition that by using a flexible construction to connect the mask to the frame, it is possible to construct a mask-frame combination which overcomes the doming difficulties in a colour display tube incorporating a substantially flat shadow mask. The flexible connection between the frame and the mask will compensate for differences in expansion between the mask and the frame. The mounting construction of the mask-frame assembly in the panel will also correct the misregistrations that will occur if the mask is heated as a consequence of incident electrons.
The invention is described using a frame that comprises folded pipes, however this should not be seen as a limitation of the invention. The frame may equally well comprise, for instance, extruded pipes or solid bars as supporting frame parts.
A preferred embodiment of the colour selection means according to the present invention is characterized in that the pipes of the frame are weak in terms of torsion properties. In a symmetrical construction, the frame parts only have to be stiff upon bending if a mask is stretched over the frame. This means that no severe demands have to be made with respect to the torsion properties of such a frame part, which makes the construction easier.
A further embodiment of the colour selection means according to the present invention is characterized in that the connection means comprise lugs connecting the mask to the frame. The mask is connected to the frame by a construction that allows the mask to shift in the direction of the frame pipe to which it is connected. This direction is transverse to the direction in which the mask is stretched. By virtue of this construction, differences in expansion between the mask and the frame are compensated for.
A further embodiment of the colour selection means according to the present invention is characterized in that the connection means comprise a resilient element connecting the mask to the frame. The mask is connected to the frame by a construction that allows the mask to shift with respect to the frame in the two directions of the plane of the mask which are perpendicular to the tube axis. This construction ensures that differences in expansion between the mask and the frame are compensated for. The advantage of resilient elements over lugs is demonstrated by the fact that in the case of resilient elements also the differences in expansion between the mask and the frame in the stretching direction are compensated.
A further embodiment of the colour selection means according to the present invention is characterized in that the mask comprises xe2x80x98blind edgesxe2x80x99, having a plurality of areas of reduced thickness. The presence of areas of reduced thickness is advantageous to compensate an effect which is referred to as called transverse contraction. When a mask is stretched under tension the blind edges cause discontinuous stretch in a direction which is more or less transverse to the stretching direction, which leads to undesired tensions and even wrinkles in the mask. This problem may be overcome by partly removing mask material from the regions of the blind edges.